Field termination plates for charged particle analyzers

ABSTRACT

A field termination plates is provided for charged particle analysis apparatus wherein the plate includes an electrical insulator substrate which has a surface thereof a plurality of narrow width conductive strips arranged to correspond to equipotential lines. Over a portion of a surface a band of high resistance material is deposited in contact with and transverse to said conductive strips. Over the entire surface including the conductive strips and the band of high resistance material is deposited a very high resistance electrically conductive material.

United States Patent 1 1 3,818,228 Palrnberg June 18, 1974 FIELD TERMINATION PLATES FOR 3,735,128 5/1973 Palmberg 250/305 CHARGED PARTICLE ANALYZERS 3,739,170 6/1973 Bohn et al. 250/305 Inventor: Paul W. Palmberg, Minneapolis,

Minn.

Assignee: Physical Electronics Industries, Inc.,

Edina, Minn.

Filed: Aug. 17, 1973 Appl. N0.: 389,329

US. Cl 250/305, 250/292, 250/310 Int. Cl G0ln 23/22, GOlt 1/36 Field of Search 250/281, 290, 291, 292,

References Cited UNITED STATES PATENTS 7/1960 Hall et al.- 250/297 6/1972 Elmore 250/297 Primary ExaminerWilliam F. Lindquist Attorney, Agent, or FirmSchroeder, Siegfried, Ryan & Vidas ABSTRACT A field termination plates is provided for charged particle analysis apparatus wherein the plate includes an electrical insulator substrate which has a surface thereof a plurality of narrow width conductive strips arranged to correspond to equipotential lines. Over a 12 Claims, 6 Drawing Figures PATENTEU JUN 1 8 I974 saw a 0? Fm; .5

FIELD TERMINATION PLATES FOR CHARGED PARTICLE ANALYZERS The present invention is directed to improvements in apparatus for charged particle analysis. More specifically it is directed to a working improvement towards elimination of electrical field distortion resulting from electrode termination in such analyzers. This goal is accomplished by an improved field termination construction.

In my earlier filed application, now U.S. Pat. No. 3,735,128, entitled FIELD TERMINATION PLATE and assigned to the same assignee as the present invention there is disclosed and claimed an arrangement for markedly improving the electrical fields in charged particle analyzers. While the apparatus disclosed in my earlier filed case identified above is an improvement over prior apparatus, it still did not provide the desired freedom from field fringing for the most precise analyses. In particular, when complex configurations were utilized for the field termination plates, difficulties were encountered in achieving the proper electrical fields.

The present invention constitutes a distinct improvement over my earlier patent above both in the ease of manufacture and in the final product resulting therefrom. While it will be described with particularity in connection with a concentric tube analyzer it will be readily apparent that the principles of my invention are similarly applicable to other forms of electric field apparatus.

In any type of charged particle device the optimum performance requires that the electrical field between the source point and the image point be free of distortion. The electrical field is generated by at least two electrodes which usually have a high degree of symmetry and which bear a precise relationship to one another. For practical reasons, such as sample accessability, it is usually necessary to terminate the field forming conducting electrodes. If the field distortion caused by this termination extends into the region of the charged particle trajectories, the performance of the analyzer will be impaired. Complete removal of field distortion at the termination can be achieved if a surface between the electrodes at their ends is established such that the potential at each point on this surface is identical to that in the same point of space when the electrodes are not terminated.

The invention will be best understood from a study of the following description and drawings wherein:

IN THE DRAWINGS:

FIG. 1 is a side cross sectional view of a coaxial cylinder analyzer with field termination plates in accordance with the invention;

FIG. 2 is a front elevational view of a field termination plate of the analyzer of FIG. 1;

FIG. 3 is a cross sectional along line 33 of FIG. 2;

FIG. 3a is a cross sectional view of an alternative form of field termination plate for the analyzer of FIG. 1;

FIG. 4 is an enlarged front elevation view of a partly fabricated plate in accordance with the invention illustrating in schematic manner trimming of the first transverse band resistance layer; and,

FIG. 5 is a cross sectional view of a quadrupole field termination plate in accordance with the invention.

The invention will be described, as it is applied in the case of a concentric tube cylindrical analyzer, as illustrated in FIG. 1. As previously noted it should be understood that the invention is similarly applicable to other types of charged particle analyzers.

Referring now to FIG. 1, 10 generally designates a cylindrical tube analyzer of the type described and claimed in U.S. Pat. No. to Bohn et al., 3,739,170 entitled AUGER ELECTRON SPECTROSCOPY and assigned to the same assignee as the present invention.

The analyzer generally designated 10 consists of an outer metallic tube 11 (typically stainless steel) which has positioned internally thereof a second cylindrical stainless steel tube 12 in axial alignment with tube 11. Tube 12 is provided with a plurality of openings 13 and 14 extending around the tube 12 adjacent opposite ends thereof. These openings 13 and 14 are partially closed by a metallic screen which permits charged particles to pass therethrough but generally-provides continuity of the field forming electrode 12.

Mounted in the intermediate region between openings 13 and 14 is an electron gun 15which is utilized in bombarding a target 17 of the material to be analyzed with high energy electrons. While the drawing shows the electron gun being coaxial with tubes 11 and 12 and intermediate openings 13 and 14, it is also useful to position the electron gun in the alternate position shown exterior to the target material so as to provide a grazing incidence for the electrons striking target 17. When one uses a grazing incidence it is desirable to have at least the sample side end plate 16 in the shape of a truncated cone as illustrated in FIGS. 1 and 3. This construction provides close positioning of the electron source or of other apparatus to be used to the ends of the analyzer and accordingly the sample being analyzed is desirably positioned relative to openings 13.

Supporting target 17 is a mounting means schematically illustrated as 18 for centrally positioning the target so that secondary electrons emitted therefrom will pass into the open end of tube 12 and at least in part thereof openings 13 and thence between electrodes 11 and 12 to be analyzed whereupon they pass outwardly through openings 14 and are impinged upon electron multiplier 19, for detection. An aperture plate 35 aids in further refining the analysis. As this general arrangement is one taught in the aforementioned patent to Bohn et al., it will not be further elaborated upon here.

Spacing tubes 11 and 12 are holding them in fixed relationship with one another is a pair of truncated hollow cones 16 which also providesmeans for achieving the proper electric field within the region between tubes 11 and 12 that makes possible the improved analysis in accordance with the invention. The members 16 will be referred to as plate members. They may take a variety of shapes. As illustrated in FIG. '1, bothof members 16 are truncated cones.'Of course, one canhave a flat disc as shown in FIG. 3a at the electron multiplier end of the assembly and still have the advantages of the truncated cone construction at the sample end.

As shown in FIGS. 3 and 3a, the plates may for apparatus of the type shown in FIG. lbe either a truncated cone or in the form of a flat disc. For other apparatus the shape may be quite different and still retain the concepts of the present invention.

The interior surface of member 16 is provided as best seen in FIGS. 2 and 3 with a plurality of annular ring members through 24 of a high conductivity material such as metal. While various metals are suitable sputtered gold-chromium is preferred. Rings 20 through 24 are desirably of narrow width and thickness. A width of about 0.005 inches and a thickness of about 0.001 inches is acceptable. Narrower and thinner cross section are also possible with the principal consideration being that the electrical conductivity of the rings be high enough to insure that each ring is essentially equipotential over its entire circumference. For the sake of clarity in the drawings, both the width and the thickness of the metal rings has been exaggerated. Over the surface of the ceramic plate 16 and rings 20 through 24 there is deposited a high resistivity coating of a suitable material which may be a cermet although virtually any high resistivity material that will withstand the conditions of the analysis (primarily high vacuum and some elevated temperature) is acceptable. This arrangement has been described and claimed in my U.S. Pat. No. 3,735,128 entitled FIELD TERMINATION PLATE assigned to the same assignee as the present invention. As set forth in my aforementioned patent, the cylinders 11 and 12 form two plates of an electric field generating means. In the central regions between the two cylinder ends the electrical field will be as desired. However, due to the fact that the cylinders are not infinite in length, there will be field fringing adjacent each end of the cylinders.

In accordance with the invention of my U.S. Pat. No. 2,735,l28, annular electrically conductive rings 20 through 24 are provided on the interior face of each of the plates 16 and the entire interiorly facing assembly is covered with a high resistance although electrically conducting material. The high resistance material provides a very low conductivity path between the two cylinders and acts to bleed off any charge build-up on the surface of the ceramic. Such bleeding off is to prevent charging of the surface of the ceramic from changing the desired field and to thereby insure that the potential of any point on members 16 is the same as the equivalent spatial point in the interior of the analyzer if the cylinders were infinitely long.

The terms conductive with respect to rings 20 through 24 and high resistance are relative terms. The high resistance coating will desirably have a resistivity on the order of 1 megohm or higher. Rings 20 through 24 will have comparatively low resistivity.

It is difficult to deposit a film such that local regions of the coating will not have greater or less conductivity than the desired amount. Therefore, by inclusion of rings analogous to rings 20 through 24, a non-uniform region of coating 27 which adversely affects the desired voltage drop between the cylinders is periodically corrected as the rings 20 through 24 being of high conductivity will insure that at each ring the potential will be the same around the circumference of the plate. The conductive rings 20 through 24 assure that one has cylindrical symmetry.

The present invention improves upon the invention disclosed and claimed in my aforementioned patent by also insuring the correct radial function so that each ring is not only equipotential but is also at the potential of the equivalent spatial ring in the interior of the analyzer if the cylinders were infinitely long.

On any surface having substantial size it is difficult to form a high resistance coating which is uniform throughout. Where the plate members 16 are truncated cone shaped as illustrated in FIGS. 1 and 3, it is very difficult to provide a uniform high resistance coating between the inner most edge at opening 25 and the outer most edge at ring 20 in the disc 16. Whether the high resistance coating is deposited by the vapor deposition by sputtering or by silk screen techniques, the same problem of lack of uniformity of coating exists. While the rings correct any cylindrical assymmetry in the manner previously described, the potential at an individual ring is determined by this voltage drop across the coating between rings. Thus, even with the control provided by the material 27, the rings may not have the desired potential if the resistance between rings due to layer 27 is not carefully controlled.

By the present invention a modification of the construction and manufacturing techniques of the aforementioned copending application have largely overcome this problem in the following manner. A ceramic disc 16 is prepared to act as a field termination plate by first depositing the annular rings 20 through 24 over the interior facing surface of the plate. This can be conveniently done by metallizing the entire interior surface and then by photoetching techniques dissolving the undesired metal to leave the rings which assure cylindrical symmetry.

Next a sector of a high resistivity material is deposited as at 26 over a portion of the interior face of plate 16 and over a portion of each of the rings 20 through 24. The high resistivity material deposited at 26 will desirably have a resistivity of approximately one megohm per square. Material 26 provides the principal conduction path between rings and therefore determines the potential at each ring. Over the top of material 26 and the balance of the face of plate 16 is deposited a second and much higher resistivity material 27 which fills in potential points between rings and bleeds off charge. Material 27 may conveniently be a megohm resistivity per square material. As the total resistance between rings is in accordance with the formula i/R llR l/R the more highly conductive layer 26 will have the principal effect on the voltage drop between consecutive rings with the very high resistivity layer 27 having only a minor effect. It is desirable to have a large ratio of resistivity between layers 26 and 27. It should be at least 10:1. The resistance provided by material 26 can be more readily controlled as it may be applied using area control such as masking.

To achieve even greater accuracy the procedure schematically shown in FIG. 4 may be followed. In FIG. 4 a region of plate 16 has had a material 26 deposited over and transverse to rings 20 24 as shown in a sector-like arrangement. Following deposition a fine sandblaster 28 is utilized in conjunction with an ohm meter 31 to trim the resistive layer between consecutive rings to a desired predetermined resistance. For best results it is desirable to have removal of material on a radial line. as shown, in narrow increments keeping the border parallel to a field line until the desired resistance is obtained between consecutive rings of metal.

After the resistance layer 26 has been trimmed a layer 27 of higher resistivity is deposited over the face of plate 16 in the same manner as previously described. The very high resistivity of layer 27 fills in potential points and allows any charges which appear on the surface of plate 16 to be bled off although layer 27 contributes very little to the voltage drop between consecutive rings. Layer 26 is the main source of control for this drop.

Various materials and machines can be used in producing the present invention. The plate members are desirably made of a non-conductive material such as a ceramic. Alumina is a useful material for plates 16. Various resistive materials can be used for layers 26 and 27. A cermet such as described in US. Pat. No. 3,739,170 may be used or commercially available resistive coatings may also be used. The coating should be compatible with the high vacuum and high bake-out temperatures encountered when the analyzer is in use. The thickness of a deposited coating 26 will typically be less than one mil.

Trimming of layer 26 may be conveniently accomplished by use of a resistor trimming system available from S. S. White Industrial Products Co. of New York, New York, under their model No. LAT-100.

As a final step in manufacture of the terminal plate of the invention, layer 27 is deposited over the surface of plate 16 including metal rings 20 24 and trimmed resistor 26. A sputtered thin film of a cermet of nickel metal and alumina is suitable. As the trimmed resistor 26 has the dominant effect on reducing fringing the criticality formerly existing for depositing layer 27 no longer exists although it is still desirable to control the uniformity of layer 27.

As an alternative to sand blast trimming of resistor 26, it has been found that chemical etching may be used. One manner by which chemical etching is used is to measure the resistance between consecutive rings so as to determine the deviation from the desired value. Then by use of a mild etching agent the region to be trimmed is chemically etched to reduce its thickness by the desired amount. By timing of the etch period a close control can be obtained for the final resistance.

As already indicated the invention is not limited to the specific form illustrated. As an example of an alternative use of the invention, attention is directed to FIG. 5 of the drawings wherein there is shown a cross sectional view of a quadrupole field termination unit of the type well known in the art. As the quadrupole per se is of known construction and purpose, it will not be discussed further herein. For discussing the prior art quadrupoles the reader's attention is directed to texts on the subject.

In FIG. 5, a view is shown of the face of a field terminating plate for quadrupole; the plate being constructed in accordance with the present invention. Referring to the figure, four rod members 50 which are arrayed in rectangular configuration and terminate in the face of a ceramic, or other electrically insulating material, plate 51. Means (not shown) are provided for impressing the desired voltages to rods 50. An opening 52 is provided in the center portion of plate 51 for introducing charged particles which traverse the length of the quadrupole.

The surface of plate 51 has had produced on the interior face thereof a plurality of conductive strips 53, 54, and 55 whose position on the face of plate 51 corresponds to the equipotential regions at least within the regions between and defined by poles 50. As shown the strips forming the conductive equipotential lines are joined at a region remote from the central portion of plate 51.

At one region of the plate 51 where the equipotential lines 53, 54 and 55 extend outside of the active region of the quadrupole a section of high resistivity material 56 has been deposited onto plate 51 and in contact with and over the surfaces of lines 53, 54, and 55. This region 56 corresponds to region 26 of FIGS. 2 and 4 in construction and purpose. It may, as in the discussion relating to FIG. 4, be trimmed to produce the desired voltage drop and thus potential at lines 53, 54 and 55 respectively.

Over the surface of plate 51 including region 56 there is then deposited a much higher resistivity layer corresponding to layer 27 of FIGS. 2 4 in construction and purposes.

The principals of the invention having been illustrated, it will now be apparent to those skilled in the art that the invention will be applicable to many constructions where it is desired to eliminate or at least markedly reduce field distortion due to fringing.

What is claimed is:

1. An electric field assembly comprising:

a. first and second conductive members defining a space therebetween;

b. an electrically non-conductive member positioned adjacent at least one corresponding end of said first and second conductive members to provide a surface facing inwardly between said conductive members, said non-conductive member defining a field termination plate;

. a plurality of spaced narrow width, low resistance electrically conducting strips positioned on said non-conductive member inwardly facing surface and spaced thereon in paths corresponding to an equipotential line between said first and second conductive members when said members are of infinite length;

d. a first layer of high resistance electrically conductive material on a portion of the inward face of said non-conducting member in electrical contact with and extending transversely across said strips; and,

e. a second layer of very high resistance electrically conductive material extending over the entire interiorly facing surface of said non-conducting member including said strips and said first layer.

2. An assembly in accordance with claim 1 wherein the ratio of the resistivities of said first and second layers is greater than 10:1.

3. An assembly in accordance with claim 1 wherein said first layer has a resistivity of about one megohm per square and said second layer has a resistivity of about megohm per square.

4. An assembly in accordance with claim 2 wherein said conducting strips are metal and are covered by said layers.

5. An assembly in accordance with claim 1 wherein said first and second conductive members are metal cylinders coaxially arranged with respect to one another with the inner cylinder having an annular opening in the wall thereof adjacent each end thereof and said non-conductive member is annular in configuration with one non-conductive member positioned adjacent each end of said cylinders holding them in spaced relationship to one another.

6. An assembly in accordance with claim wherein said non-conductive members are hollow truncated cones.

7. An assembly in accordance with claim 5 wherein said conductive strips are narrow rings of metal coaxial to the axis of said cylinders.

8. An assembly in accordance with claim 5 wherein said first layer is deposited over a sector of each of said annular non-conducting members and said second layer is deposited over said first layer.

9. An end disc for minimization of electrical field fringing in a cylindrical analyzer for charged particles comprising:

a. an annular shaped plate member constructed of of a non-conductive material;

b. a plurality of spaced narrow width electrical conducting strips on said non-conductive plate coaxially arranged with respect to the axis of said nonconductive plate and with respect to one another;

e. a first layer of high resistance electrically conductive material on a sector of the surface of said nonconductive plate and extending transversely across and in electrical contact with said strips;

f. a second layer of very high resistance electrically conducting material overlying said first layer and the balance of the surface of said non-conducting plate and said strips.

10. An end disc in accordance with claim 9 wherein the ratio of the resistivity of said second layer to said first layer is greater than 10:1.

11. An end disc in accordance with claim 9 wherein said plate is a hollow truncated cone.

12. An end disc in accordance with claim ll wherein said first layer has a resistivity of about one megohm per square and said second layer has a resistivity of about megohms per square, 

1. An electric field assembly comprising: a. first and second conductive members defining a space therebetween; b. an electrically non-conductive member positioned adjacent at least one corresponding end of said first and second conductive members to provide a surface facing inwardly between said conductive members, said non-conductive member defining a field termination plate; c. a plurality of spaced narrow width, low resistance electrically conducting strips positioned on said nonconductive member inwardly facing surface and spaced thereon in paths correspondiNg to an equipotential line between said first and second conductive members when said members are of infinite length; d. a first layer of high resistance electrically conductive material on a portion of the inward face of said non-conducting member in electrical contact with and extending transversely across said strips; and, e. a second layer of very high resistance electrically conductive material extending over the entire interiorly facing surface of said non-conducting member including said strips and said first layer.
 2. An assembly in accordance with claim 1 wherein the ratio of the resistivities of said first and second layers is greater than 10:1.
 3. An assembly in accordance with claim 1 wherein said first layer has a resistivity of about one megohm per square and said second layer has a resistivity of about 100 megohm per square.
 4. An assembly in accordance with claim 2 wherein said conducting strips are metal and are covered by said layers.
 5. An assembly in accordance with claim 1 wherein said first and second conductive members are metal cylinders coaxially arranged with respect to one another with the inner cylinder having an annular opening in the wall thereof adjacent each end thereof and said non-conductive member is annular in configuration with one non-conductive member positioned adjacent each end of said cylinders holding them in spaced relationship to one another.
 6. An assembly in accordance with claim 5 wherein said non-conductive members are hollow truncated cones.
 7. An assembly in accordance with claim 5 wherein said conductive strips are narrow rings of metal coaxial to the axis of said cylinders.
 8. An assembly in accordance with claim 5 wherein said first layer is deposited over a sector of each of said annular non-conducting members and said second layer is deposited over said first layer.
 9. An end disc for minimization of electrical field fringing in a cylindrical analyzer for charged particles comprising: a. an annular shaped plate member constructed of of a non-conductive material; b. a plurality of spaced narrow width electrical conducting strips on said non-conductive plate coaxially arranged with respect to the axis of said non-conductive plate and with respect to one another; e. a first layer of high resistance electrically conductive material on a sector of the surface of said non-conductive plate and extending transversely across and in electrical contact with said strips; f. a second layer of very high resistance electrically conducting material overlying said first layer and the balance of the surface of said non-conducting plate and said strips.
 10. An end disc in accordance with claim 9 wherein the ratio of the resistivity of said second layer to said first layer is greater than 10:1.
 11. An end disc in accordance with claim 9 wherein said plate is a hollow truncated cone.
 12. An end disc in accordance with claim 11 wherein said first layer has a resistivity of about one megohm per square and said second layer has a resistivity of about 100 megohms per square. 